If you're running many virtualised instances of Debian, using something like UML or Xen, you may see large performance hits during the day. This may be due to I/O intensive cron-jobs being launched on multiple instances at the same time.

You need to stagger the times for cron jobs. The first thing you need to do is edit the /etc/crontab on each of the instances. Modify the "minutes" field to something other than the default on each of the systems.

 
17 *    * * *   root    run-parts --report /etc/cron.hourly
25 6    * * *   root    test -x /usr/sbin/anacron || run-parts --report /etc/cron.daily
47 6    * * 7   root    test -x /usr/sbin/anacron || run-parts --report /etc/cron.weekly
52 6    1 * *   root    test -x /usr/sbin/anacron || run-parts --report /etc/cron.monthly

Then have a look at the files in /etc/cron.d/ . There may some such as "logcheck" that will benefit from being shifted a few minutes.

It's a shame that the crontab doesn't support some fuzzy/random feature to timed execution. Hopefully, if it ever evolves to eat cron, that's the sort of thing upstart will support.